1. Field of the Invention
The present invention relates to a crystal oscillator, and more particularly relates to a fundamental wave/overtone crystal oscillator enabling one crystal oscillator to oscillate in a fundamental wave mode or in an overtone mode.
2. Description of the Related Art
Prior Art
Conventionally, a crystal unit is capable of oscillating in a fundamental wave mode or in an overtone mode with common specifications.
However, a circuit has to be configured in an optimum condition for each of the fundamental wave mode and the overtone mode, and furthermore an excitation current is preferably set optimally for usages and specifications of a crystal unit. Therefore, when a plurality of signals for fundamental waves and for overtones are to be obtained in one device, the device has to be provided with a pair of a crystal unit and a circuit for each mode.
[Fundamental Wave Crystal Oscillator: FIG. 7]
Referring to FIG. 7, a conventional fundamental wave crystal oscillator is described below. FIG. 7 is a circuit diagram of a conventional fundamental wave crystal oscillator.
As illustrated in FIG. 7, the conventional fundamental wave crystal oscillator includes a crystal unit 1a oscillating with fundamental waves and an oscillator circuit 10.
The oscillator circuit 10 is made up of elements optimized for fundamental waves.
The oscillator circuit 10 is a Colpitts oscillator circuit, in which an excitation current of a crystal unit 1a is fed to the base of a transistor Tr11 via a capacitor C13, and a power supply voltage Vcc is supplied to the collector of the transistor Tr11 as well as the base via a resistor R11.
The collector of the transistor Tr11 is connected with one end of a capacitor C14, and the other end of the capacitor C14 is grounded.
The base of the transistor Tr11 is further connected with a capacitor C11 and a capacitor C12 connected in series, and the other end of the capacitor C12 is grounded.
The emitter of the transistor Tr11 is grounded via a resistor R12.
A point between the capacitor C11 and the capacitor C12 is connected with the emitter, and the emitter outputs an oscillatory frequency for fundamental waves.
[Overtone Crystal Oscillator: FIG. 8]
Referring to FIG. 8, a conventional overtone crystal oscillator is described below. FIG. 8 is a circuit diagram of a conventional overtone crystal oscillator.
As illustrated in FIG. 8, the conventional overtone crystal oscillator includes a crystal unit 1b oscillating with overtones, and an oscillator circuit 20.
The oscillator circuit 20 is made up of elements optimized for overtones.
The oscillator circuit 20 is a Colpitts oscillator circuit, in which an excitation current of a crystal unit 1b is fed to the base of a transistor Tr21 via a capacitor C23, and a power supply voltage Vcc is supplied to the collector of the transistor Tr21 as well as the base via a resistor R21.
The collector of the transistor Tr21 is connected with one end of a capacitor C24, and the other end of the capacitor C24 is grounded.
The base of the transistor Tr21 is further connected with a capacitor C21 and a capacitor C22 connected in series, and the other end of the capacitor C22 is grounded. The emitter of the transistor Tr21 is grounded via a resistor R22.
A point between the capacitor C21 and the capacitor C22 is connected with the emitter, and the emitter outputs an oscillatory frequency for overtones.
[Negative Resistance Characteristics: FIG. 9, FIG. 10]
Referring next to FIG. 9 and FIG. 10, negative resistance characteristics in crystal oscillators are described below. FIG. 9 exemplifies a negative resistance characteristic of an oscillator circuit with fundamental waves, and FIG. 10 exemplifies a negative resistance characteristic of an oscillator circuit with overtones.
Oscillation occurs when the circuit side viewed from the crystal unit shows negative resistance at an oscillatory frequency of the crystal unit.
Oscillation occurs with fundamental waves in the circuit having negative resistance in a fundamental wave frequency band. FIG. 9 exemplifies negative resistance in the fundamental-wave frequency band for a crystal unit of 26 MHz.
Oscillation occurs with overtones in the circuit having negative resistance in an overtone band. FIG. 10 exemplifies negative resistance in an overtone frequency band for a crystal unit of 26 MHz.
Related Art
Related art includes: Japanese Patent Application Laid-Open No. S64-051806 “IC for crystal oscillator” (Applicant: Nihon Dempa Kogyo Co., Ltd./Patent Document 1), Japanese Patent Application Laid-Open No. 2004-128593 “Oscillation circuit” (Applicant: Kinseki Ltd/Patent Document 2), and Japanese Patent Application Laid-Open No. 2009-177393 “Third overtone oscillator circuit” (Applicant: NEC Corp/Patent Document 3).
Patent Document 1 discloses an IC for crystal oscillator, in which a first resistor and a second resister having resistance lower than that of the first resistor are disposed between an input terminal and an output terminal, and in the case of fundamental wave oscillation, the first resistor is selected, and in the case of the third overtones, the second resistor is selected by a switch 7, thus flexibly performing switching between fundamental waves and the third overtone oscillation.
Patent Document 2 discloses an oscillation circuit including two feedback resistors connected in parallel with an inverter and a piezoelectric resonator connected between the input and the output terminals of the inverter, in which any one of the feedback resistors is selected by a switching element, thus performing switching between fundamental wave oscillation and overtone oscillation.
Patent Document 3 discloses a third overtone oscillator circuit in which a crystal unit is connected in parallel with an inverter, and a plurality of capacitors and switches for selection are provided for each of the input terminal and the output terminal. Even when the negative resistance characteristic of the oscillator circuit changes with a stray capacitance of a board, the capacitance of the capacitors are adjusted by switching the switches, thus adjusting the negative resistance.